remove ubuntu-20 #1166

Job	Run time
iOS (aarch64-apple-ios, iphoneos)	13s
iOS (x86_64-apple-ios, iphonesimulator)	14s
Windows AArch64	1m 5s
Build Targets (wasm32-unknown-unknown)	17s
Build Targets (wasm32-wasip1)	15s
Build Targets (x86_64-unknown-fuchsia)	17s
Docker (aarch64-unknown-linux-gnu)	38s
Build Targets (x86_64-fortanix-unknown-sgx)	17s
Docker (arm-unknown-linux-gnueabihf)	42s
Miri	2m 30s
Build Targets (x86_64-unknown-illumos)	17s
Docker (armv7-unknown-linux-gnueabihf)	37s
Docker (i586-unknown-linux-gnu)	28s
Docker (i686-unknown-linux-gnu)	30s
MSRV (ubuntu-24.04)	15s
Rustfmt	3s
Docker (powerpc64-unknown-linux-gnu)	35s
MSRV (windows-latest)	31s
Docker (s390x-unknown-linux-gnu)	37s
Docker (x86_64-pc-windows-gnu)	38s
Test (ubuntu-24.04, stable)	1m 54s
Docker (x86_64-unknown-linux-gnu)	27s
Test (ubuntu-24.04, beta)	2m 5s
Docker (x86_64-unknown-linux-musl)	28s
Test (ubuntu-24.04, nightly)	2m 19s
Docker (arm-linux-androideabi)	1m 16s
Test (macos-latest, stable)	1m 4s
Docker (armv7-linux-androideabi)	1m 20s
Test (macos-latest, nightly)	1m 31s
Docker (aarch64-linux-android)	1m 28s
Test (windows-latest, stable-x86_64-msvc)	1m 29s
Docker (i686-linux-android)	1m 17s
Test (windows-latest, stable-i686-msvc)	1m 56s
Docker (x86_64-linux-android)	1m 17s
Test (windows-latest, stable-x86_64-gnu)	2m 32s
Test (windows-latest, nightly-x86_64-msvc)	1m 22s
Test (windows-latest, nightly-i686-msvc)	2m 3s
Test (windows-latest, nightly-x86_64-gnu)	2m 21s
	39m 8s

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